The present invention relates to a transmittable light-scattering sheet (or film) useful for assuring a high-luminance display of images in a liquid crystal display unit (in particular, reflective liquid crystal display unit), a method of producing the same, and a reflective liquid crystal display unit utilizing the light-scattering sheet.
Recent years have witnessed advances in the construction of infrastructures for telecommunications systems and in the network consolidation of information through the computer-communications equipment integration, such as internets. By such a network consolidation, the access to information is freed from restrictions as to time and place. In order to utilize such networks efficiently, portable information terminals such as PDA (personal digital assistance) have been developed. Moreover, in lieu of notebook-sized personal computers, further-downsized mobile personal computers of reduced thickness and weight are under development.
Since portability is required of these units, it is necessary to reconcile the need for a longer battery operating time with the need for reduced thickness and downsizing of communications units. Therefore, display for use in such portable telecommunications equipment must be of reduced thickness, reduced weight and low power consumption. Particularly for attaining the low power consumption goal, a technology for brightening the display screen by exploiting natural light has been sought for replacing the conventional technology using a backlight. The display mode which is considered to be most promising is a reflecting liquid crystal display unit. Particularly, to keep abreast with the increasing versatility of data accompanying the ever-continuing advances in multimedia, there is a demand for reflecting-mode liquid crystal display units not only capable of large color display and high image-quality (high-definition) display but also of low production cost.
As the reflecting mode liquid crystal display unit, there are known various kinds of units such as TN (Twisted Nematic) and STN (Super Twisted Nematic) units, but for color display and high-definition display, units utilizing a polarizer (one polarizer plate type) are preferred. For example, the R-OCB mode in which the liquid crystal layer is of the HAN (Hybrid Aligned Nematic) alignment has excellent characteristics such as low voltage, wider viewing angle, high-speed response, middle color rendition and high contrast.
In order to insure the uniform brightness of the screen as accompanied with getting a display larger, the scattering function is an important factor. That is, in the reflective liquid crystal display unit, the brightness of the screen is insured in such manner that the light incident on the liquid crystal layer (natural light, ambient light) is efficiently taken in and reflected with a reflector, that the reflected light is scattered to an extent not deteriorating visibility for the prevention of total reflection. When the polarizer and light-scattering sheet are combined, the reflection efficiency can be further improved. Incidentally, as the polarizer, there can be used a light-reflecting back electrode utilizing an electrode as a reflector, and a reflecting plate disposed outside of a support of an electrode. For example, as the reflecting type liquid crystal display unit, in Japanese Patent Application Laid-Open No. 228887/1988 (JP-63-228887A) and Photofabrication Symposium ""92 sponsored by the Japanese Society of Printing, the fundamental technology about liquid crystal display unit, and the liquid crystal display unit given an enlarged viewing angle of the display surface through the prevention of total reflection by means of adopting a surface-corrugated metal thin film as the back electrode (lower electrode) were introduced.
However, when the display of the reflecting type liquid crystal display unit is to be a color display, a color filter is used in addition to said polarizer. In case a color filter is used, the proportion of loss of reflected light is increased and the above scattering plate system cannot impart enough brightness to the display screen. Particularly, in the color system, it is particularly important to impart high luminance by directing diffused light in a given direction (directed scattering). However, in order to increase this directionality in the scattering-reflector system, it is necessary to precisely control the geometry and distribution pattern of reflector surface irregularities but this is a costly procedure.
For the purpose of insuring a high luminance by scattering reflected light, there has been known a liquid crystal display unit using a transmittable light-scattering sheet in lieu of a scattering reflector. For example, Japanese Patent Publication No. 8430/1986 (JP-61-8430B) discloses a liquid crystal display unit comprising a polarizing layer formed on the front side of a liquid crystal cell and, as formed thereon, a light-scattering layer. However, a resin sheet as polymerized by utilizing holography for imparting directionality to a transmitting type light-scattering sheet is known (The synopsis of Lectures at Japanese Society of Liquid Crystal Science, 1998) but the production process is complicated and costly.
Meanwhile, as a light-scattering sheet with low cost also known is a particle dispersion sheet having an islands-in-an ocean structure composed of resin beads or spheres and a transparent resin. Japanese Patent Application Laid-Open No. 261171/1995 (JP-7-261171A) discloses a display unit having a light scattering layer externally of a liquid cell, specifically a display unit comprising a polarizing film on the outer surface of an electrode plate and, as formed on the surface of the polarizing film, a light-scattering layer comprising a phase-separated dispersion of two or more kinds of resins varying in refractive index. As disclosed in Japanese Patent Application Laid-Open No. 27904/1995 (JP-7-27904A) and Japanese Patent Application Laid-Open No. 113902/1997 (JP-9-113902A), there also are known transmitting type liquid crystal display units such that a particle-scattering sheet having an islands-in-an ocean structure comprised of plastic beads and a transparent resin matrix is interposed between a back light and a liquid crystal cell. As an example of the display unit having a light-scattering layer within the liquid crystal cell, Japanese Patent Application Laid-Open No. 98452/1995 (JP-7-98452A) discloses a display unit comprising a transparent resin layer (light-scattering layer) containing a dispersion of fine particles as interposed between the electrode and substrate sheet (electrode-supporting substrate) of the electrode plate.
However, in these islands-in-an ocean structure sheets, since the resin beads are dispersed randomly in a transparent resin matrix, the scattering light intensity distributes according to Gaussian distribution in principle. Thus, the directionality can not imparted to the light-scattering light, and it is difficult that the brightness is imparted to the display surface. Particularly, in the reflective liquid crystal display unit having a large display screen the particle dispersed sheet can not impart sufficient brightness to the whole display screen and it is difficult to ensure sharp image.
It is, therefore, an object of the present invention to provide a light-scattering sheet (or film) having high light-scattering properties and capable of imparting directionality to a transmitted and scattered light, a process for producing the light-scattering sheet and a liquid crystal display unit (in particular, reflective liquid crystal display unit) with the use of the light-scattering sheet.
It is another object of the present invention to provide a light-scattering sheet capable of scattering a transmitted light with high directionality, a process for producing the light-scattering sheet, and a liquid crystal display unit (in particular, reflective liquid crystal display unit).
It is still another object of the present invention to provide a light-scattering sheet capable of ensuring high image-quality even in a color display, a process for producing the light-scattering sheet, and a liquid crystal display unit.
It is other object of the present invention to provide a light-scattering sheet capable of assuring high brightness and high precise display of images, a process for producing the light-scattering sheet, and a liquid crystal display unit.
It is yet other object of the present invention to provide a process for producing the light-scattering sheet with convenient and low cost.
The inventors of the present invention did much research to accomplish the above objects and found that by removing or evaporating a solvent from a liquid mixture composed of a plurality of resins varying in refractive index to cause spinodal decomposition, a bicontinuous phase structure which has the regularlity and is substantially isotropic can be easily formed, and that a transmitted light can be scattered with high directionality when the sheet having such a bicontinuous phase structure is used. The present invention has been developed on the basis of the above findings.
Thus, a light-scattering sheet of the present invention comprises a light-scattering layer which comprises a plurality of polymers varying in refractive index and has at least bicontinuous phase structure, wherein the bicontinuous phase structure is formed by spinodal decomposition from liquid phase comprising a plurality of polymers. In the sheet, an incident light can be scattered isotropically, and the transmitted and scattered light may have a maximum intensity of a scattered light at a scattering angle of 2 to 40xc2x0 and a high total light transmittance (for example, 70 to 100%). In the light-scattering sheet, an average interphase distance of the bicontinuous phase may be about 0.5 to 20 xcexcm, and a difference in refractive index between a plurality of polymers may be about 0.01 to 0.2.
The light-scattering sheet may comprise a transparent support and the light-scattering layer formed on at least one side of the support. In the sheet, the transparent support is optically isotropic.
The present invention also includes a process for forming the light-scattering sheet by removing a solvent from a layer composed of a plurality of polymers varying in refractive index and the solvent to form a bicontinuous phase structure due to spinodal decomposition. In the process, the mixture may be applied on a transparent support and a solvent in the mixture may be evaporated to form a bicontinuous phase structure.
The present invention also includes a reflective liquid crystal display unit which comprises a liquid crystal cell having a liquid crystal sealed therein, a reflecting means for reflecting an incident light disposed behind the liquid crystal cell, and the light-scattering sheet disposed forwardly of the reflecting means. In this unit, a polarizing plate may be disposed forwardly of the liquid crystal cell, and a light-scattering sheet may be disposed between the liquid crystal cell and the polarizing plate.
Throughout this specification, the term xe2x80x9csheetsxe2x80x9d means, without regard to thickness, a dimensional material thus meaning a film as well.